1 Design of Open Channels and Culverts CE453 Lecture 25 Ref: Chapter 17 of your text and HYDRAULIC DESIGN OF HIGHWAY CULVERTS, Hydraulic Design Series Number 5, Federal Highway Administration, Publication No. FHWA- NHI-01-020, September 2001; available at http://www.cflhd.gov/design/hyd/hds5_03r.pdf , accessed March 18, 2006
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Design of Open Channels and Culverts
CE453 Lecture 25
Ref: Chapter 17 of your text and HYDRAULIC DESIGN OF HIGHWAY CULVERTS, Hydraulic Design Series Number 5, Federal Highway Administration, Publication No. FHWA-NHI-01-020, September 2001; available at http://www.cflhd.gov/design/hyd/hds5_03r.pdf, accessed March 18, 2006
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Design of Open Channels
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Longitudinal Slopes Gradient
longitudinal direction of highway to facilitate movement of water along roadway
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Drains
Along ROW Collect surface water
A typical intercepting drain placed in the impervious zone
http://www.big-o.com/constr/hel-cor.htm
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Drainage Channels (Ditches)
Design Adequate capacity Minimize hazard to traffic Hydraulic efficiency Ease of maintenance
Desirable design (for safety): flat slopes, broad bottom, and liberal rounding
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Ditch Shape
Trapezoidal – generally preferred considering hydraulics, maintenance, and safety
Source: Fabriform1.com
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Ditch Shape
V-shaped – less desirable from safety point of view and maintenance
Source: Fabriform1.com
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Terms
Steady Flow: rate of discharge does not vary with time (Manning’s applies)
Uniform: channel properties are constant along length of channel
SlopeRoughnessCross-section
Water surface is parallel to slope of channel
Non-uniform: properties vary
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Terms
Unsteady flow: rate of discharge varies with time Critical depth
a hydraulic control in design depth of water where flow changes from tranquil to rapid/shooting
Critical velocity: velocity corresponding to critical depthCritical slope: slope corresponding to critical depth
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Flow Velocity
• Depends on lining type • Should be high enough to prevent deposit
of transported material (sedimentation)• For most linings, problem if S < 1% (generally
velocity should be > 2 fps when full)• Should be low enough to prevent erosion
(scour)• For most types of linings, problem if S > 5%
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Use spillway or chute if Δelev is large
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Rip Rap for drainage over high slope
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Riprap (TN Design Manual)
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Side Ditch/Open Channel Design-Basics
• Estimate Q at point of interest• Select ditch cross section• Erosion control?• Manning’s formula for design• Assume steady flow in a uniform
channel
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Manning’s Formula
V = R2/3*S1/2 (metric) V = 1.486 R2/3*S1/2 n n
where: V = mean velocity (m/sec or ft/sec)R = hydraulic radius (m, ft) = area of the cross section of flow (m2, ft2) divided by wetted perimeter (m,ft)S = slope of channeln = Manning’s roughness coefficient
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Side Ditch/Open Channel Design-Basics
Q = VAQ = discharge (ft3/sec, m3/sec)A = area of flow cross section (ft2, m2)
FHWA has developed chartsto solve Manning’s equation for different cross sections